Sales dude? Did he do any diagnostics apart from retrieving the codes?
I would not jump to conclusion and start replacing components without any diagnostics to confirm the cause of the problem.
DTC P0420 : CATALYTIC SYSTEM EFFICIENCY BELOW THRESHOLD
If DTCs P0136, P0137, P0138, P0139 and/or P0141 are set, trouble shoot those DTCs before trouble shooting DTC P0420.
Perform PCM reset procedure. Start engine and run at 3000 RPM with no loads and transmission in Park or Neutral, until radiator fan comes on. Allow engine to idle. Connect SCS service connector. Test drive vehicle at 40-55 MPH for approximately 2 minutes. Decelerate
for at least 3 seconds with throttle fully closed. Reduce vehicle speed to 35 MPH, and hold it there until readiness code comes on. If DTC P0420 is not indicated, problem is intermittent, system is okay at this time. If DTC P0420 is indicated, inspect Warm-Up Three Way Catalytic converter (TWC), if equipped and/or Three-Way Catalytic Converter (TWC) for plugging, melting, cracking, exhaust system leaks. Repair or
replace as necessary.
Sunday, January 6th, 2013 AT 7:46 PM